When "regular" items, such as list items, documents, or libraries or lists, get deleted from a SharePoint site, they go to the end user (aka first stage) recycle bin, and their size continues to count against the quota. If they are deleted from the end user recycle bin (or if they are auto-deleted after 30 days), then they stop counting against the quota. This can be verified on the "Site Collection Quotas and Locks" page in Central Admin.
When a site (not site collection) gets deleted, it skips the first stage recycle bin and goes straight to the 2nd stage recycle bin. However, its size appears to continue counting against the site quota. (Again, verified in Central Admin.)
Will the site size continue to count against the quota until it is removed from the second stage recycle bin, either by manual removal or auto-removal after thirty days, or will a timer job reduce the reported usage?